It felt natural to me for Rich's initial reaction to this revelation to be poor.
Part of the inspiration for this was simply that I've listened to a lot of people's experiences playing roleplaying games, and one thing that has come up several times has been favoritism for players who the Game Master was either dating, or interested in dating.
This favoritism could take the form of that player effectively being made the story's main protagonist. The plot revolves around them, the unique gear was created for them, etc.
So while Rich lacks tact in that first panel (and in general), his reaction wasn't out of nowhere. He could easily have been reminded of such things.
Fortunately for everyone at the table, the only reason Nanase's character has this unusual origin is to accommodate her being unfamiliar with everything, and that's simply provided an opportune explanation for Rich's paladin being invested in the party.